3) if not so, splunk not guarantee db
connect 2.0.5 to user oralce 10g
Oracle 10g went out of extended support in July 2013. Oracle's "Sustaining support" model does not allow for newly created updates to Oracle 10g and it does not allow for "Certifications with new third party products / versions". ( See http://www.oracle.com/us/support/library/lifetime-support-technology-069183.pdf ). Oracle 10g is effectively end of life. While I can't speak for Splunk as a company (I do not work for them), I would think asking them to "guarantee" compatibility with an end of life third party product is a bit of a large request.
I don't see any reason why DB Connect would NOT work with Oracle 10g, but you are going to have to do the work to make it work. You'll need to get a compatible JDBC driver and configure DB Connect to use it.